We are a customer with about all of the Microsoft licenses that one can buy, but I have a service request that's been open for about three weeks without even being assigned to an agent. I opened it through the Microsoft 365 Admin Center. How do I get action taken here?
If you have a Unified Support agreement then you should have a CSAM, they are listed in the Alternate Contacts in the Services Hub (https://serviceshub.microsoft.com/gethelp). If you are working with a Microsoft Partner then they can also escalate SRs or raise them on your behalf..